The preparation and construction of a building foundation is an elaborate process that involves setting up footing forms to below frost line, and building a concrete wall on top of the footing to above ground level on which will rest the walls of the building. The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the soil and adjacent materials, and on nearby heat sources. The line varies by latitude; it is deeper closer to the poles. It ranges in the United States from about zero to six feet.
A footing installed above the frost line may heave as ground below it freezes and thaws in cold weather wracking and twisting the structure that is built upon it. An unprotected footing in the US may need to be placed as much as 72″ below ground level to protect the footing from wracking due to the freezing and expansion of the earth that supports the footing. Usually the steps for building the footing and foundation take several days, large excavation machinery and extensive excavation to achieve the appropriate excavation width and footing depth below frost line even if it is for a footing for an outside building such as a garage, greenhouse or sunroom.
Footing excavations generally need to be 30″-36″ wide in order to accommodate construction of the typical 16″-24″ wide by 8-10″ high footing. The concrete foundation walls that will support the above ground structure are then formed and poured on top of the new footing after the footing has cured for typically a couple of days. These foundation walls are generally 6″ to 8″ thick and run around the entire perimeter of the structure.